Robert Baratheon was the first king of the Baratheon dynasty to sit on the Iron Throne of Westeros.
Early life
Robert was the eldest son of Lord Steffon Baratheon and his wife Lady Cassana, of House Estermont. Robert's parents died in a shipwreck in Shipbreaker Bay when he was still a teenager, and he became Lord of Storm's End at a young age. His parents were returning from Essos when they were caught in a sudden storm. Robert and Stannis were standing on the walls of Storm's End and witnessed thir parents ship breaking up on the rocks.
Robert rose to the occasion, with the solid and reliable support of his brother Stannis, although they were not personally close. Robert also had to raise his youngest brother Renly, but he handed over this task to the castle's staff.
He was born and raised in the Baratheon stronghold of Storm's End, but at a young age was sent to foster with Lord Jon Arryn at the Eyrie in the Vale of Arryn. There he met and befriended Eddard Stark, who had also been sent to foster there by his father. They also befriended a foreigner who had washed ashore in the Vale in a storm, Alek Nocher.
Despite their radically different temperaments, Robert and Eddard became close and loyal friends. Jon Arryn, who had no sons of his own, came to treat them as his own family, and both Robert and Eddard came to regard the Eyrie as their second home, visiting often even after they had left and returned home.
Robert fell in love with Eddard's sister Lyanna, and her father Lord Rickard approved the high-profile match. However, shortly after this event Lyanna was apparently kidnapped and taken against her will from Winterfell by Prince Rhaegar Targaryen.
Robert's Rebellion
News of the kidnap was slow to reach the Eyrie, where Robert and Eddard were staying at the time, and by the time they learned it Eddard and Lyanna's eldest brother Brandon Stark had already gone to King's Landing to confront Rhaegar. Rhaegar was not present, but a furious King Aerys II had him arrested for threatening the life of the prince and summoned Lord Rickard to the capital. Lord Jon exercised caution and prevented Robert and Eddard from joining Rickard. Rickard and his retainers, along with Brandon and his companions (including Jon Arryn's nephew and heir, Elbert), were executed and Aerys put a call out for Robert and Eddard to also present themselves for justice.
Instead, Jon, Eddard (now Lord of Winterfell), Alek and Robert raised the standards of rebellion. Houses Arryn, Baratheon and Stark summoned their banners and Jon Arryn made Alek a Knight of the Vale just prior them all leaving for war. House Tully had also been done insult, as Brandon had been betrothed to Lord Hoster's daughter Catelyn. Some hard and fast politicking was done by Lord Jon and it was agreed that Catelyn would marry Eddard and Hoster's eldest daughter Lysa would marry Jon. In exchange Hoster Tully joined his forces to those of the rebellion.
The Stark, Tully and Arryn armies began assembling north of the Trident, but the Baratheons were separated from their allies and surrounded by enemies. Robert defeated several other houses and won them to his cause through sheer charisma, but marching his army north to link up with the rest was extremely hazardous, as it involved passing close to Tyrell territory, and they remained loyal to the king. His troops succeeded in the task (although Storm's End came under siege from the Tyrell armies), although Robert was briefly separated from his men and almost killed at the Battle of Stoney Sept until Eddard's forces rescued him.
United, the formidable army of the rebels marched south and smashed the royalist forces at the Battle of the Trident, where Robert slew Prince Rhaegar in combat with a blow of his war hammer. Robert sustained some wounds in the process and had to send Eddard to attack King's Landing.
Whilst the rebel army bore down on King's Landing from the north, the armies of House Lannister arrived to reinforce the capital. Once the gates had been opened, they instead sacked the city. Aerys was slain by Ser Jaime Lannister and the city was surrendered to the rebel forces once they arrived.
To ensure there was no threat to Robert's line, Lord Tywin Lannister had arranged the deaths of Rhaegar's children, which Eddard constituted unjustified murder, leading to a schism between him and Robert that was only healed when they learned of Lyanna's death.
Claiming the Iron Throne
It is unclear when Robert decided to claim the Iron Throne directly. Although he had decided that Aerys and Rhaegar had to die for their crimes, it is unknown if the fate of their heirs had similarly been decided. Eddard's reaction suggests maybe not.
Robert's claim to the Iron Throne was based on the marriage of his grandfather to Princess Rhaelle Targaryen, the daughter of King Aegon V, thus making Robert the legal heir to the Targaryen dynasty if Aerys and all his descendants were killed. This gave Robert a much stronger claim to the throne than anyone in the Arryn, Tully or Stark camps. After Aerys and Rhaegar's deaths, and with Aerys' remaining children surrounded and effectively imprisoned on Dragonstone, the remaining lords of the Seven Kingdoms agreed to swear fealty to Robert and accept him as King.
Early rule
Robert began his reign by marrying Cersei Lannister, in thanks for Tywin's service in delivering the capital to him. Robert had no wish to marry any woman after the death of Lyanna, but Jon Arryn convinced him that a king must have heirs and Cersei Lannister would insure her father's loyalty and the Wealthy House Lannister. The marriage was not happy, and although Cersei bore three children (Joffrey, Myrcella and Tommen), Robert was not especially satisfied. He did not like his eldest son and heir Joffrey, beating him for once cutting open a pregnant cat to see what her insides looked like, and in his unhappiness took to drink, feasting and sleeping with other women. In this way he had several bastard children, most notably Mya Stone, Edric Storm and Gendry, a blacksmith's apprentice in the capital.
Robert appointed Lord Jon Arryn as the Hand of the King, and Jon was an able and effective administrator, repairing relations with Dorne which had turned sour after Lannister bannermen slew Princess Elia Martell during the Sack and preventing another war.
The Greyjoy Rebellion
Six years after the war, Lord Balon Greyjoy of the Iron Islands declared independence from the Seven Kingdoms, with the ironborn attacking and destroying the Lannister fleet at Lannisport and sacking Seagard to forestall any attacks on their territory. Greyjoy believed that Robert's response would be weak, as befitting a traitor and usurper, but instead Robert rallied a substantial force and fleet against him. In a pitched battle at Pyke the castle was taken. Both of Balon's elder sons died in the war and his youngest, Theon, was taken as hostage by Eddard Stark. Balon brought in chains to Robert by Alek Nocher was forced to swear fealty once again to the Iron Throne.
Later rule
The swift and brutal repression of the Greyjoy rebellion earned Robert much respect, but this dwindled as he spent far too much of the realm's coin on feasting and tourneys. Without an enemy to fight, he went to seed and became noticeably fat and tired. When Lord Jon Arryn died, he appointed Eddard Stark as his new Hand. Robert saw that he was surrounded by schemers out for themselves and he had no idea who to trust anymore.
Death
Robert was injured after hunting boar in the Kingswood. He had too much to drink, and Cersei had arranged for the wine to be overly strong. Thus he was not in good shape to be hunting such a beast. He survived a few days after the injury, but died in the Red Keep, naming Eddard as Regent until Joffrey came of age.
Unfortunately, Robert was not aware that Eddard had already discovered that Joffrey, Myrcella and Tommen were the products of incest between Queen Cersei and her twin brother Jaime. Eddard had warned Cersei to flee with her children from Roberts wrath, which prompted her to arrange Roberts death. Eddard had decided not to mar Roberts last hours with talk of his wifes betrayal. He thought that as regent with the support of the gold cloaks, he could seize the throne and deliver it to Stannis as the rightful heir. Eddard was betrayed, his men were killed and he was arrested and executed at Joffrey's order. Robert's brother Stannis, who had learned the truth, also claimed the Iron Throne, as did his youngest brother Renly who had been promised the support of the Tyrells. With the Iron Islands, North and the Riverlands rising in rebellion as well, Robert's death triggered the beginning of the War of the Five Kings.
Character and appearance
As a younger man, Robert was proud, tall, commanding and fierce in battle. Immensely charasmatic, he won people to his side and to his cause through sheer personal magnetism. He was never happier than fighting at the lists or on the battlefield. He celebrated hard as well, but as a younger man always took care to maintain his peak fitness. He had no love of politicking or intrigue.
Some of the joy went out of Robert upon the death of Lyanna, and this only increased as his loveless marriage with Cersei continued. With the battles and wars won, and with his family and Kingsguard disapproving of him taking part in tourneys and melees, Robert lost his former vigour and became fat and somewhat indolent. Some of his legendary humour left him, and his temper came out more and more often, especially if the Targaryen name was mentioned. Aside from Jon Arryn and Alek Nocher, he had few true friends at court. He sometimes joked that he would like to leave everything behind to become a hedge knight. In the end, he found that winning a kingdom was far more fun than actually ruling one.